LADAR imaging technology compared to traditional Radar technology enjoys a higher resolution due to the shorter wavelengths of light compared to RF and therefore is useful in a number of applications, which include precision target selection, automatic target recognition, agile laser designators, imaging IR seeker illuminators, 3D imaging for small unmanned airborne systems (UAS), vehicle adaptive cruise control systems, autonomous navigation systems, and speed and hazard detection for vehicle collision avoidance systems. The chip-scale scanning and coherent IR LADAR receiver concept disclosed herein has advantages of a higher Signal to Noise Ratio (SNR) due to its scanning and coherent detection capabilities. Also, because of lower atmospheric losses in the IR (and more particularly at longer IR wavelengths), the receiver SNR improves further. The ability to operate at room temperatures means that no external cooling of the components is necessary, resulting in a lower cost of manufacturing. A the chip scale size results in lower costs, a smaller size and lower power requirements compared to conventional LADAR technologies.
There is no prior art concerning a chip-scale IR frequency modulated LADAR receiver that the inventors are presently aware. However, there is prior art in the area of mid-IR coherent Doppler LADAR and near-IR coherent frequency modulator continuous wave (FMCW) LADAR which the presently disclosed concept significantly improves upon as explained above. None of this art
Frequency-modulated continuous-wave LADAR (FMCW)—also called continuous-wave frequency-modulated (CWFM) LADAR—is a short-range measuring radar set capable of determining distance. This increases reliability by providing distance measurement along with speed measurement, which is important when there is more than one source of reflection arriving at the radar's antenna. This kind of LADAR could be used as “LADAR altimeter” to measure the exact height during the landing procedure of aircraft, for example.
A state-of-the-art mid-IR coherent LADAR receiver, which was developed by NASA Langley Research Center, operates at a wavelength of 2 μm and is based on an optical fiber system with discrete components. See, “High Energy Double-pulsed Ho:Tm:YLF Laser Amplifier”, Jirong Yu, NASA Langley Research Center, Laser System Branch, MS 474, Hampton, Va. 23681. It uses a bulky solid-state Ho:Tm:YLF laser as the local oscillator, and dual-balanced InGaAs photodiodes for optical detection. The limitations of this LADAR receiver system are: (1) fixed wavelength operation determined by the solid-state laser material, (2) bulky, heavy, high power consuming and expensive system, (3) the use of discrete cascaded optical components results in higher overall noise and loss, (4) limitation in the wavelength of mid-IR detected signal to λ<2.5 μm due to use of InGaAs photodiodes, and (4) use of high detectivity mid-IR photodetectors for wavelengths longer than 3 μm would require cooling.
The proposed chip-scale mid-IR coherent LADAR receiver concept disclosed herein has orders of magnitude lower size, weight and power (SWAP), has lower noise, is wavelength selectable in the long to mid-IR portions of the IR spectrum (and preferably from 2-12 μm) and can be monolithically integrated with room temperature highly sensitive Si avalanche photodiodes, as well as with CMOS electronics for post processing of the detected IR signal.
There is also prior art in a non-coherent, direct detection mid-IR LADAR operating at 3.4-3.5 μm wavelengths. This LADAR system is also optical fiber based and uses an interband cascade laser as the optical source and a TE-cooled HgCdTe photodetector in its receiver. See, “New Developments in HgCdTe APDs and LADAR Receivers”, Proc. SPIE 8012, Infrared Technology and Applications XXXVII, 801230 (Jun. 20, 2011).
Finally, there is also prior art in a near-IR coherent FMCW LADAR operating at 13 μm wavelength. This LADAR system is also optical fiber based and uses a diode-pumped Nd:YAG laser as the optical source and balanced InGaAs photodetectors for the detection of the received LADAR signal. See, for example, “Chirped Lidar Using Simplified Homodyne Detection”, Journal of Lightwave Technology, Vol. 27, p. 3351, 2009.
In summary, the state-of-the-art IR LADAR transceivers are based on bulky modules that use discrete cascaded components which result in higher noise and optical loss, required cooling for improved photodetector performance at longer IR wavelengths and do not have electronic scanning capability.